Intussusception of the bowel is one thing that could be investigated or ruled out.
more info. on that here:
https://www.acvs.org/small-animal/intussusception
Another thing is possible chronic enteritis...
Geez, I do not know what this could be. I worked for a vet 8 years, and never seen THAT. I think you should just observe her - is she mostly happy or mostly miserable? If she is mostly unhappy, consider that you may have to do the hardest and most loving thing you can do for her, euthanasia. I'm sorry to say that. But I do think it is important to think about.
What scan did the vet do? An ultrasound, or MRI?
Is it definite that there is no evidence of disk problems? Was an X ray done? Or MRI of her spine?
Poor sweet girl. So young and having to bear so much pain on a regular basis ever since she was a tiny pup....
Screaming with pain is a huge deal for a dog. They usually bear chronic pain with a stoical approach, though when in pain may refuse to eat or be lethargic. But to scream in pain during these 'attacks' means the level of pain must be high.
I am not a qualified vet. I'd just like to make that clear. Only an ordinary person with some knowledge of dogs and a wish to be of help if possible, if and when I can. But I don't know what this could be.
Prednisone is for short term treatment only. It sound as if it is just putting a band-aid on the situation, because she is still getting these attacks, so that means it isn't anywhere near solved.
Obviously, a pain killing medicine would be a good idea to have as a stand-by to use during an attack. A liquid form would be best, if you can get that from the vet! - then it can be gently syringed into the side of her mouth, as when in great pain she will probably not be able to eat food with a pill in it.
I know there are a few vets on the team at your clinic, but another idea would be to take her for a second opinion??
IBS is a possibility, yes. But I have no idea how that would be diagnosed. But does that cause acute pain? I have no knowledge of what it's like to have IBS.
Developing mange is a horrible extra for her. But typical when the immune system is lowered.
Your poor girl. I felt like crying for her, and I WISH I had better suggestions!